1. Field of the Invention
The present invention relates to apparatus for the noncontact measurement of the profile of a surface. More particularly, the invention relates to optical apparatus which is useful for the high accuracy measurement of surface roughness or of the height of a step change in a thickness of an opaque film on a substrate.
2. The Prior Art
Prior art techniques available for measuring the profile of a surface include mechanical and optical profilers. A commonly used contacting apparatus used to measure surface profiles and step heights is a stylus instrument, e.g., the Talysurf or the Talystep. However, in the case of a soft or delicate surface, the stylus digs into the surface and measurement results do not truly represent the surface. Other limitations of the stylus technique include its high sensitivity to microphonics and vibrations, the delicate nature of the stylus and the mechanism, and the need for a highly skilled operator to align and use it.
There are numerous optical techniques available for measuring the profile of a surface. For a review and comparison of some of the more common techniques, see J. M. Bennett, "Comparison of Techniques for Measuring the Roughness of Optical Surfaces," Optical Engineering, Vol. 24, No. 3, pp. 380-387, 1985.
Prior art optical profilers have been based on a variety of techniques, e.g., scanning fringes of equal chromatic order (FECO) interferometry, see for example, J. M. Bennett, "Measurement of the RMS Roughness, Autocovariance Function and Other Statistical Properties of Optical Surfaces using a FECO Scanning Interferometer," Applied Optics, Vol. 15, pp. 2705-2721 (1976); scanning Fizeau interferometry, see for example, J. M. Eastman and P. W. Baumeister, "Measurement of the Microtopography of Optical Surfaces using a Scanning Fizeau Interferometer," J. Opt. Soc. Am., Vol. 64, p. 1369 (A) (1974); optical heterodyne interferometry, see for example, G. E. Sommargren, "Optical Heterodyne Profilometry," Applied Optics, Vol. 20, pp. 610-618 (1981); a Mirau interferometer, see for example, B. Bhushan, J. C. Wyant, and C. L. Koliopoulis, "Measurement of Surface Topography of Magnetic Tapes by Mirau Interferometry," Applied Optics, Vol. 24, pp. 1489-1497 (1985), and J. C. Wyant and K. N. Prettyjohns, U.S. Pat. No. 4,639,139, issued January 27, 1987; a Nomarski-based instrument, see for example, S. N. Jabr, "Surface-roughness measurement by digital processing of Nomarski phase contrast images," Optics Letters, Vol. 10, pp. 526-528 (1985); a birefringent microscope, see for example, M. J. Downs, U.S. Pat. No. 4,534,649, issued August 13, 1985; and shearing interference microscopy, see for example, M. Adachi and K. Yasaka, "Roughness measurement using a shearing interference microscope," Applied Optics, Vol. 25, pp. 764-768 (1986).
FECO interferometry requires that the test surface be brought very close to the reference surface, e.g., typically within about several micrometers, thereby frequently causing the test surface to be damaged by residual dust particles.
The optical heterodyne interferometer which is both common path and does not require a reference surface produces very accurate and precise measurements. While this technique provides state-of-the-art optical measurements, it suffers from a number of limitations. In particular, the apparatus is complex and expensive. In addition, since the technique only scans in a circle of fixed radius, it does not profile an area of the test surface.
The conventional white light or filtered white light Mirau type interferometer suffers from several serious limitations. First, since a beamsplitter and reference surface must be placed between the objective lens and the test surface, only mid-range objective lens magnifications can be used. Second, the central obscuration caused by the placement of the reference surface in the beam path of the imaged wavefronts adversely affects the image contrast of mid-range spatial frequencies. Third, due to the presence of these optics between the objective lens and the test surface, an extended light source is required. With a conventional light source, the coherence length is thusly limited to 3-6 micrometers. This short coherence length leads not only to a very tight vertical alignment tolerance for the test surface to obtain interference fringes, but also limits the amount of tilt and curvature of the test surface which can be measured. Other two-beam, equal path interferometer microscopes such as the Michelson and Linnik when used with an extended incoherent illumination source suffer the same tight vertical alignment tolerance as does the Mirau interferometer microscope.
The birefringent microscope technique is both common path and does not require a reference surface. However, it does have some severe limitations. First, it only scans a line so that it does not profile an area of the test surface. Second, it is limited in its ability to use a sufficiently large diameter for the reference beam on the test surface, thereby limiting the extent to which lower spatial frequencies can be measured.
In the present invention, high precision profile measurements can be made wherein the interference of a two-beam microscope interferometer is localized within the full depth of focus of every microscope objective lens magnification permitting a large vertical alignment tolerance. Large test surface tilt, out of plane separation of features, and curvature can be tolerated relative to the prior art. Low magnification (i.e., 1X to 5X) objectives can now utilize the Mirau type interferometer configuration. Improved mid-range spatial frequency response for all magnifications is realized without the central obscuration of the prior art. The improvements of the present invention, thusly, overcome the disadvantages of the prior art and allow the high accuracy, fine lateral resolution measurement of surface microroughness profiles and step heights.